1. Field of the Invention
This invention relates to a process for oxidizing the vanadium and uranium contained in wet process phosphoric acid to a higher valence state for extraction by subsequent contact with complex organic extractants. This invention further relates to an electrolytic cell in which to practice the process. The cell is capable of operating at relatively high current efficiency over extended periods of time with little or no maintenance.
2. Description of the Prior Art
Phosphoric acid is generally produced commercially by one of two methods. One method is generally called the "furnace" method, and the other is generally referred to as the "wet process" method. In the wet process method of producing phosphoric acid, phosphate rock is contacted with a mineral acid such as sulfuric acid. Most phosphate rock contains metal compounds in varying amounts. In many cases, these metal compounds are dissolved from the phosphate rock and appear in the wet process acid as contaminants. Vanadium and uranium compounds are among those dissolved from the phosphate rock, particularly when the rock is from the so-called western deposits of Idaho, Wyoming, Utah and Montana.
Vanadium, however, is an undesirable component of wet process phosphoric acid in that it prevents the use of the phosphoric acid in making animal feed supplements, as the amount of vanadium must be kept at a low level in such animal feed supplements. Similarly, uranium is undesirable in such feed supplements. Further, the removal of these uranium values from wet process phosphoric acid would prevent their release to the environment via the widespread usage of the fertilizer end products.
On the other hand, vanadium and uranium are valuable materials and wet process phosphoric acid can be an important source of these materials. Therefore, a process for the recovery of vanadium and uranium from wet process phosphoric acid provides an important benefit in that the vanadium and uranium are converted from undesirable contaminants to valuable by-products.
There are processes for the simultaneous coextraction of vanadium and uranium from aqueous acidic solutions containing the same, such as for example the process disclosed by U.S. Pat. No. 3,836,476. There also are numerous processes for the recovery of vanadium or uranium separately from wet process acids.
In the coextraction processes and many of the other processes, it is necessary to oxidize the vanadium and uranium to a higher valence level to enable the vandium and uranium to be separated.
In the past, this oxidation has been achieved through the addition of chemical oxidants in an amount in excess of the stoichiometric requirement for oxidizing all the vanadium to vanadium (V) and all the uranium to uranium (VI). Normally, the oxidant is added in an amount of from about 50 percent to 1000 percent in excess of that which is stoichiometrically required. Various oxidizing agents have been used such as chlorates, manganese dioxide, permanganates, dichromates, peroxydisulfates, and ceric salts.
The disadvantages resulting from chemical oxidation are increased process expense, the possibility of increased plant corrosion, and further contamination of the acid through addition of the oxidizing agent.
Though while not applied to the oxidation of vanadium and uranium, it is known that phosphoric acid containing trivalent iron impurities can be purified by direct current electrolytic reduction of the iron to the divalent oxidation state and precipitation of the iron impurities by the process disclosed in U.S. Pat. No. 2,288,752, or by direct current electrolytic reduction of the iron impurities and recovery of the phosphoric acid by extraction with a water-insoluble amine extractant, such as, disclosed in U.S. Pat. No. 3,479,139.
The principal disadvantage of these reduction processes is that they operate by passing a constant direct current through the phosphoric acid electrolyte to purify the material. Thus, the prior processes do not provide a means of controlling the electrolysis, resulting in decreased current efficiency and thereby increased operating expenses.
It is desirable to provide a process whereby vanadium and uranium may be oxidized automatically by electrolytic means with improved current efficiency.